A scanning method using a plurality of types of different tube voltages, that is, multi-energy CT scanning has been developed for X-ray computed tomography imaging apparatuses. Multi-energy CT scanning includes a scanning method using two tube voltages, that is, dual-energy CT scanning. An application of dual-energy CT scanning is raw data decomposition of generating two types of raw data (to be referred to as reference substance data hereinafter) about two reference substances set in advance by using raw data derived from X-rays generated at a low tube voltage, and raw data derived from X-rays generated at a high tube voltage. Data of an image (to be referred to as a reference substance image hereinafter) targeting the first reference substance is generated based on the first reference substance data, and the second reference substance image data is generated based on the second reference substance data.
In raw data decomposition, a known X-ray spectrum is used. Ideally, the energy spectrum of X-rays generated from an X-ray tube should be constant. In practice, however, the X-ray spectrum sometimes shifts by about several kV. Owing to the discrepancy of an actual X-ray energy spectrum from an ideal X-ray energy spectrum, the accuracy of raw data decomposition degrades. As a result, the qualities of a reference substance image, a monochromatic X-ray image and density image based on the reference substance, and an effective atomic number image degrade.
In image reconstruction in a normal scanning method using a single tube voltage (to be referred to as single-energy CT scanning hereinafter), scaling processing is performed to suppress the discrepancy of an actual X-ray energy spectrum from an ideal X-ray energy spectrum. When scaling processing in single-energy CT scanning is applied to dual-energy CT scanning, the scaling processing is performed at the time of image reconstruction after raw data decomposition. However, raw data after raw data decomposition does not have CT value information, so the scaling processing in single-energy CT scanning cannot be applied to dual-energy CT scanning.
An object of an embodiment is to provide an X-ray computed tomography imaging apparatus and photon counting CT apparatus capable of suppressing deterioration of the image quality arising from the discrepancy of an actual X-ray energy spectrum from an ideal X-ray energy spectrum.